1. Technical Field
The present relates to a method for producing patterned metal nanowires, electrode using the patterned metal nanowires, and transistor using the patterned metal nanowire electrode.
2. Description of Related Art
With the advancement of technology, the last two decades have seen the emergence of a plethora of computers, communication products, and consumer electronics. In the meantime, it has been an important goal for the designers and manufacturers of many of the display-based electronic products to develop materials featuring both low electrical resistance and high light permeability, and because of that, materials meeting the foregoing criteria such as carbon nanotubes (CNTs), graphene, metal oxides, and metal nanowires are receiving more and more attention. Taking indium tin oxide (ITO)—which has found wide application in transparent conductors—for example, it has outstanding electronic and optical properties and is frequently used in various electronic devices. In particular, ITO plays a crucial role in the development of electronic displays and solar cells. However, the shortage of indium and the high costs of related production equipment have driven scientists to search for alternative materials.
Metal nanowires, especially silver nanowires (AgNWs), are the most promising material for use in transparent conductors in place of ITO. As silver has the highest electrical conductivity of all metals (about 100 times as conductive as ITO), intertwined silver nanowires in a conductor form a highly conductive network, which, due to the high aspect ratio (length/diameter ratio) of silver nanowires, is thinly structured and hence extremely light-permeable (i.e., having a considerable transmission rate) in addition to being highly conductive. Transparent conductors made with silver nanowires have high conductivity and excellent light permeability and are therefore suitable for use as touchscreen sensors or as anodes in an OLED (organic light-emitting diode) lighting system. Besides, transparent conductors made with silver nanowires can serve as pixel electrodes in an electronic display or as the upper and lower electrode plates in a solar cell. Since Microsoft's Windows 8 operating system became commercially available, touchscreens have been incorporated into laptop, desktop, and all-in-one (AIO) computers and become the trend of the market. Most touchscreens are based on a projected capacitance design, which requires high-quality transparent conductors in order to provide satisfactory user experience. Apart from being extensively used in tablet, laptop, and desktop computers and mobile phones, touchscreens are also applied to interactive kiosks, game consoles, POS (point-of-sale) terminals, automobiles, the Global Positioning System (GPS), and many other consumer electronics. Touchscreens, it seems, have turned out to be the most intuitive of all user interfaces. Now that both OLED lighting and organic solar cells demand super high conductivity (e.g., sheet resistance lower than 20 Ω/sq) and exceptional light permeability (e.g., transmission rate higher than 90%), silver nanowires are replacing ITO, which fails to meet those requirements, and have been used by not a few manufacturers in their latest product lines.
In order to respond in real time to ten-point touch control—a feature of the recently developed large touchscreen displays (e.g., 23-inch touchscreens), high conductivity and high-resolution patterning become a must. As metal nanowires are randomly distributed, they do not form visually interfering patterns or Moire patterns and are therefore conducive to better visual effects. Meanwhile, it is generally desired that the touchscreen of a mobile device (e.g., a laptop computer) be lighter, thinner, and more durable, and in response to that, the market demand for plastic films integrated with conductors is exceeding that for the conventional glass substrates. In addition, the successful development of flexible displays has made it necessary to adapt transparent conductors to non-planar designs. That is to say, transparent conductors nowadays must be pliable and rollable as well as patternable. More importantly, given the declining prices of consumer electronics, the costs of transparent conductors must follow suit. Compared with other transparent conductors, those made with silver nanowires can better satisfy the above requirements.
There have been researches on how to pattern metal nanowires (especially silver nanowires) on polymer substrates. Three of the conventional techniques are briefly stated below by way of example.
Prior art technique 1: The “partial etching” process of Cambrios Technologies of US and Toray involves cutting the silver nanowires in the etching area with laser but not removing the cut silver nanowires. Thus, with the cut silver nanowires still present but not in electrical conduction with one another, patterning is achieved.
Prior art technique 2: The Direct Printing Technology (DPT) of Gunze of Japan involves making ink with silver particles whose sizes range from several dozen to several hundred nanometers and forming a silver grid by printing with the ink through a high-precision screen, wherein the silver grid is composed of square openings defined by lines which are 20 μm wide and arranged at an interval of 300˜1,000 μm. As the silver grid directly forms the pattern of a touch sensor during the printing process, neither electroplating nor etching is needed, and this technique is applicable to a roll-to-roll process. DPT can make DPT films printed with different sensor patterns, and the films can be used in the form of a single sheet or two sheets stacked together.
Prior art technique 3: Generally, a silver nanowire-containing liquid is applied through a slit to form a roll, which is subsequently patterned by lithography. The entire process, however, is very complicated. Hitachi Chemical of Japan proposed the Transparent Conductive Transfer Film (TCTF) technique, which, published in Finetech Japan, is a new-generation method for making transparent electrode films by a transfer process. The film structure thus produced includes a cover film as the top layer, a 5 μm-thick photoresist film and a transparent conductive layer as the middle layer, and a carrier film as the bottom layer, wherein the transparent conductive layer includes silver nanowires provided by Cambrios. The TCTF method begins by transferring a TCTF to a glass substrate or a transparent plastic substrate at a temperature of 100˜140° C. Then, the film is covered with a photomask and exposed to light for the first time. After removing the carrier film, the remaining film structure is exposed to light for the second time, and a developing step (with 1% Na2CO3 aqueous solution) follows to produce an etched pattern whose depth ranges from 0.4 to 0.8 μm. Last but not least, a high-energy blanket exposure is carried out to cure the insulating layer fully, thereby completing the patterning of the transparent electrode and the bonding of the insulating layer simultaneously.